The present invention relates to a method for the thermal treatment of gypsum in which the material is dewatered in a continuous process in the fluidized bed with inbuilt heat exchange surfaces. The invention also relates to an apparatus for carrying out this method.
Moist gypsum, i.e. calcium sulfate-dihydrate to which moist gypsum from exhaust gas disulfurizing plants, so-called REA-gypsum, also belong, can be surface dried at temperatures up to about 60 to 70.degree. C. without removing the water of crystallization. At temperatures up to about 150.degree. C. the extraction of the water of crystallization takes place above all through the formation of the hemihydrate. Of this there are the alpha or beta modifications, with the beta modification mainly arising. Above these temperatures the anhydrite modifications arise, depending on the temperature level, through complete extraction of the water of crystallization, starting with the so-called AIII gypsum, a very reactive component with short setting time, followed by sluggishly reacting AIIs of poor solubility and the inert AIIu gypsum. These differences are attributed to the surface nature modified by the thermal treatment.
In order to obtain a calcined gypsum with constant and/or pure phase composition it is necessary to be able to control the temperature profile very precisely during the manufacture. No instances of overheating and inhomogeneities may be present during the thermal treatment which lead to undesired phase conversions for individual particles of the material.
In order to prevent undesired phase conversions already in the drying stage, a two-stage method for the production of calcined gypsum with adjustable phase composition is described in DE 41 09 743. In this method the drying of moist gypsum is carried out in the first stage in a fluidized bed apparatus with inbuilt heat exchange surfaces at a low temperature level. In the second stage the dried gypsum is treated in a second fluidized bed apparatus with inbuilt heat exchange surfaces at a low temperature level, in order to remove the water of crystallization. In both fluidized bed apparatuses the heat which is used is admitted with the fluidization gas and the heat exchange surfaces, with the heat being predominantly introduced via the heat exchange surfaces.
In this method the quality of the calcined gypsum is significantly improved in comparison to other previously known methods; however the purity and the constancy of the phase composition of the calcined gypsum is not yet entirely satisfactory. Moreover, in this method, a drying step is always necessary.
The difficulty in the calcining of gypsum lies in the fact that all the phases can exist alongside one another and, in distinction to other hydrates, no clear equilibrium state defined only by the temperature can be achieved in the technical apparatus as a result of inhibited reactions. The dewatering and phase formation takes place at the individual particles from the particle surface to the particle core in dependence on the temperature and the dwell time. Each particle contains the following phases during the calcining: externally anhydrite (for example AIII), in the interior dihydrate and in the intermediate region .beta.-hemihydrate. The proportions of these phases, i.e. the degree of calcining, depends on the particle size. Under the same conditions, small particles have a lower dihydrate content than large particles, i.e. a higher degree of calcining.